Considerable attention has been directed toward both gestational and senescent-related cardiovascular changes; however, little information is currently available with respect to the perinatal period. It is clear that the perinatal period encompassing weaning to early adulthood represents a time of rapid increase in cardiac hemodynamic function. Cellular and subcellular correlates of cardiac hemodynamic function have been investigated, but conclusions to date have failed to consider the heterogeneous cellular composition of cardiac tissue. Therefore, the overall objective of the proposed work is to define the characteristic cellular adaptations which occur during rat heart development from the time of weaning until early adulthood. Within the overall objective the specific aims are: 1. To determine the relative cellular, i.e. cardiac muscle cell and nonmuscle cell, composition. Total cell numbers and relative fractional cell composition will be considered. 2. To define the cellular characteristics of both cardiac muscle and non-muscle cells. Cell size, protein synthetic capacity, and indices of aerobic metabolic capability will be considered. 3. To define myocyte specific characteristics including enzymatic activity (ATPase) of contractile proteins and contractile performance determined in isolated, single cardiac myocytes. The specific aims outlined above will characterize cardiac muscle and nonmuscle cells prepared from hearts during the process of normal cell growth. From this characterization, factors which influence contractile properties of heart myocytes can be identified through complementary research areas including biochemistry and physiology. Identification of factors influencing contractile properties of normally developing heart muscle cells will serve as a useful reference for studies of altered cellular properties in various cardiac disease entities.